The Works of William Harvey M.D. / Translated from the Latin with a life of the author

by William Harvey

Nor does he wander less wide of the mark when he speaks of the chalazæ as the matter from which, by the influence of the semen galli, the chick is incorporated. For the chick is not produced either from one or the other, nor yet from both of the chalazæ, as we have shown in our history. Neither is the generation of the chick effected by metamorphosis, nor by any new form assumed and division effected in the chalazæ, but by epigenesis, in the manner already explained. Nor are the chalazæ especially fecundated by the semen of the male bird, but the cicatricula rather, or the part which we have called the eye of the egg, from which, when it enlarges, the colliquament is produced, in and from which, subsequently, the blood, the veins, and the pulsating vesicles proceed, after which the whole body is gradually formed. Moreover, on his own admission, the semen of the cock never enters the uterus of the hen, and yet it fecundates not only the eggs that are already formed, but others that are yet to be produced.

Fabricius refers the albumen and vitellus to the second action of the egg, which is the nutrition and growth of the chick. “The vitellus and albumen,” he says,[313] “are in quantity commensurate with the perfect performance of this action, and with the due development and growth of the chick. The shell and membranes are therefore the safety of the whole of the egg as well as the security of its action. But the veins and arteries which carry nourishment are organs without which the action of the egg, in other words, the growth and nutrition of the chick, would not take place.” It is uncertain, however, whether the umbilical vessels of the embryo or the veins and arteries of the mother, whence the egg is increased, are here to be understood. For a like reason the uterus and incubation ought to be referred to this last class of actions.

We have to do, then, with the two fluids of the egg, the albumen and the vitellus; for these, before all the other parts, are formed for the use of the embryo, and in them is the second action of the egg especially conspicuous.

The egg of the common hen is of two colours internally, and consists of two fluids, severally distinct, separated by membranes, and in all probability of different natures, and therefore having different ends to serve, inasmuch as they are distinguished by different extensions of the umbilical veins, one of them proceeding to the white, another to the yelk. “The yelk and white of the egg are of opposite natures,” says Aristotle,[314] “not only in colour, but also in power. For the yelk is congealed by cold; the white is not congealed, but is rather liquefied; on the contrary, the white is coagulated by heat, the yelk is not coagulated, but remains soft, unless it be over-done, and is more condensed and dried by boiling than by roasting.” The vitellus getting heated during incubation, is rendered more moist; for it becomes like melted wax or tallow, whereby it also takes up more room. For as the embryo grows, the albumen is gradually taken up and becomes inspissated; but the yelk, even when the fœtus has attained perfection, appears scarcely to have diminished in size; it is only more diffluent and moist, even when the fœtus begins to have its abdomen closed in.

Aristotle[315] gives the following reason for the diversity: “Since the bird cannot perfect her offspring within herself, she produces it along with the aliment needful to its growth in the egg. Viviparous animals again prepare the food (milk) in another part of their body, namely, the breasts. Now nature has done the same thing in the egg; but otherwise than as is generally presumed, and as Alcmæon Crotoniates states it, for it is not the albumen but the vitellus which is the milk of the egg.”

For as the fœtus of a viviparous animal draws its nourishment from the uterus whilst it is connected with its mother, like a plant by its roots from the earth; but after birth, and when it has escaped from the womb, sucks milk from the breast, and thereby continues to wax in size and strength, the chick finds the analogue of both kinds of food in the egg. So that whilst in viviparous animals the uterus exists within the parent, in oviparous the parent may rather be said to exist within the uterus (the egg). For the egg is a kind of exposed and detached uterus, and in it are included in some sort vicarious mammæ. The chick in the egg, I say, is first nourished by albumen, but afterwards, when this is consumed, by the yelk or by milk. The umbilical vascular connexion with the albumen, therefore, when this fluid is used up, withers and is interrupted when the abdomen comes to be closed, and before the period of exclusion arrives, so that it leaves no trace of its existence behind it: in viviparous animals, on the contrary, the umbilical cord is permanent in all its parts up to the moment of birth. The other canal that extends to the vitellus, however, is taken up along with this matter into the abdomen, where being stored, it serves for the support of the delicate fœtus until its beak has acquired firmness enough to seize and bruise its food, and its stomach strength sufficient to comminute and digest it; just as the young of the viviparous animal lives upon milk from the mammæ of its mother, until it is provided with teeth by which it can masticate harder food. For the vitellus is as milk to the chick, as has been already said; and the bird’s egg, as it stands in lieu both of uterus and mammæ, is furnished with two fluids of different colours, the white and the yelk.

All admit this distinction of fluids. But I, as I have already said, distinguish two albumens in the egg, kept separate by an interposed membrane, the more external of which embraces the other within it, in the same way as the yelk is surrounded by the albumen in general. I have also insisted on the diverse nature of these albumens; distinguished both by situation and their surrounding membranes, they seem in like manner calculated for different uses. Both, however, are there for ends of nutrition, the outermost, as that to which the branches of the umbilical veins are earliest distributed, being first consumed, and then the inner and thicker portion; last of all the vitellus is attacked, and by it is the chick nourished, not only till it escapes from the shell but for some time afterwards.

But upon this point we shall have more to say below, when we come to speak of the manner in which the fœtuses of viviparous animals are developed, and at the same time demonstrate that these all derive their origin from eggs, and live by a twofold albuminous food in the womb. One of these is thinner, and contained within the ovum or conception; the other is obtained by the umbilical vessels from the placenta and uterine cotyledons. The fluid of the ovum resembles a dilute albumen in colour and consistence; it is a sluggish, pellucid liquid, in all respects similar to that which we have called the colliquament of the egg, in which the embryo swims, and on which it feeds by the mouth. The fluid which the fœtus obtains from the uterine placenta by the aid of the umbilical vessels is more dense and mucaginous, like the inspissated albumen. Whence it clearly appears that the fœtus in utero is no more nourished by its parent’s blood than is the suckling afterwards, or the chick in ovo; but that it is nourished by an albuminous matter concocted in the placenta, and not unlike white of egg.

Nor is the contemplation of the Divine Providence less useful than delightful when we see nature, in her work of evolving the fœtus, furnishing it with sustenance adapted to its varying ages and powers, now more easy, by and by more difficult of digestion. For as the fœtus acquires greater powers of digesting, so is it supplied with food that is successively thicker and harder. And the same thing may be observed in the milk of animals generally: when the young creature first sees the light the milk is thinner and more easy of concoction; but in the course of time, and with increased strength in the suckling, it becomes thicker, and is more abundantly stored with caseous matter. Those flabby and delicate women, therefore, who do not nurse their own children, but give them up to the breast of another, consult their health indifferently; for mercenary nurses being for the major part of more robust and hardy frames, and their milk consequently thicker, more caseous, and difficult of digestion, it frequently happens that milk of this kind given to the infants of such parents, particularly during the time of teething, is not well borne, but gives rise to crudities and diarrhœas, to griping, vomiting, fever, epilepsy, and other formidable diseases of the like nature.

What Fabricius says,[316] and strives to bolster up by certain reasonings, of the chalazæ standing for the matter of the chick, we have already thrown out in our history, and at the same time have made it manifest that the substance of the chick and its first rudiments were produced whilst the chalazæ were still entire and unchanged, and in a totally different situation.